If you have bought 8 bags on ebay and only one has been authentic, I really really really urge you to make more use of the Authenticate This threads in the respective designer sections. Every seller is going to tell you that the bag is authentic, that is what they do, they are trying to scam you out of money.
As soon as you receive the bag, file a claim (SNAD like another tPFer suggested) and escalate it so they seller knows you mean business. If the claim is found in your favor, they will most likely ask you to destroy the bag (good riddance to a fake bag). If the scammer is a seasoned one, she may have already transferred the money out of their paypal account and you may be stuck.
Seriously, ebay is shark infested waters if you can't tell what's authentic. So if you can't, ask on the forum, no one here wants you to buy a fake bag. Some sellers do, and they will lie up and down to you and it's your hard earned money. I hope this is resolved for you.